A Japanese Patent Laid-Open No. 2009-184423 (patent document 1) described below discloses an electronic control unit, which includes a microcomputer having a normal mode (i.e., a normal operation mode) and a low power consumption mode (i.e., a standby mode) as two operation modes and a power circuit having a normal mode (i.e., a normal electric current mode) and a low power consumption mode (i.e., a low electric current mode) as two operation modes.
According to the patent document 1, the low electric power mode of the microcomputer switches to the normal mode based on an input from an external device. Further, the microcomputer outputs a mode switching signal to the power circuit for the switching of the operation modes (i.e., for the power circuit to switch to the normal mode). Therefore, the microcomputer may be re-started (i.e., wakes up) prior to the switching of the power circuit to the normal mode. To prevent an unintended operation of the microcomputer, which may be due to a voltage drop of the power circuit below an operation guarantee voltage, (i.e., to a lower voltage lower than a normal operation threshold voltage) the microcomputer may start a reset operation.
Such a restart of the microcomputer prior to the switching of the power circuit may be prevented by a capacitor that is added to an output side of the power circuit, which prevents a voltage drop of the power circuit during a switching period to switch to the normal mode. However, to prevent the restart of the microcomputer, a large capacity capacitor is required. In addition, the variation of the electric currents in the capacitor and the microcomputer has to be considered. Therefore, the production cost of the electronic control unit may be increased when the preventive measure for the restart of the microcomputer is implemented. Further, the variation of the electric currents may be caused by various reasons, such as a tolerance of the capacitor, temperature characteristics, an aging of the electronic parts or the like.